A typical UPS is one in which a primary energy source (normally a commercial AC line) and a reserve energy source (normally a battery voltage driven inverter) are both coupled to supply energy input to an energy coupling apparatus such as a transformer with two independent energy inputs. Each input is functional to supply power to a single output. In other arrangements the two sources of energy may be joined in series so that the primary source of energy charges the reserve source.
When power is being supplied from the reserve energy source, the UPS is disconnected from the commercial AC line by semiconductor electronic isolation switches located in the input line. Electronic isolation switches are normally used in place of mechanical switches in order to avoid the transients associated with the switching of mechanical switches. Should these electronic isolation switches fail in a shorted or conducting mode, power supplied by the reserve energy source may be fed back into the primary line source directly through the energy coupling apparatus. There generally is no indication of this switch failure until a current path is accidentally completed across the AC line permitting such current flow. This is a potential safety hazard which is often not apparent until such a current path is completed.